Coated abrasive articles generally contain an abrasive material, typically in the form of abrasive grains, bonded to a backing by means of one or more adhesive layers. Such articles usually take the form of sheets, discs, belts, bands, and the like, which can be adapted to be mounted on pulleys, wheels, or drums. Abrasive articles can be used for sanding, grinding, or polishing various surfaces of, for example, steel and other metals, wood, wood-like laminates, plastic, fiberglass, leather, or ceramics.
Many abrasive articles are used as discs, in grinding assemblies. A typical such abrasive sanding or grinding assembly includes: a back-up pad or support pad made from a resilient and reinforced material such as rubber or plastic; an abrasive disc, which is typically frictionally mounted on the back-up pad; and a rotatable shaft and cap for mounting the abrasive disc and back-up pad by pressure applied to the disc upon screwing the cap into the shaft so that the disc is squeezed against the back-up pad. In use, the shaft of the assembly exemplified is rotated and the abrasive coated surface of the disc is pressed against a workpiece with considerable force. Thus, the disc is subjected to severe stresses. This is also true for abrasive articles in other forms, such as belts.
The backings used in coated abrasive articles are typically made of paper, polymeric materials, cloth, nonwoven materials, vulcanized fiber, or combinations of these materials. Many of these materials are not appropriate for certain applications because they are not of sufficient strength, flexibility, or impact resistance. Some of these materials age unacceptably rapidly. In some instances the materials are sensitive to liquids which are used as coolants and cutting fluids. As a result, early failure and poor functioning can occur in certain applications.
A common material used for coated abrasive backing material is vulcanized fiber. Vulcanized fiber backings are typically heat resistant and strong, which are advantageous characteristics when the coated abrasive is used in a grinding operation that imposes severe conditions of heat and pressure. For example, vulcanized fiber is used in certain grinding operations, such as weld grinding, contour grinding, and edge grinding, wherein the coated abrasive can be exposed to temperatures greater than 140.degree. C. Vulcanized fiber backings, however, are expensive, hygroscopic, and thus sensitive to humidity.
Under extreme conditions of humidity, i.e., conditions of high and low humidity, vulcanized fiber will be affected by either expansion or shrinkage, due, respectively, to water absorption or loss. As a result, an abrasive article made of vulcanized fiber will tend to cup, causing a coated abrasive disc to curl either in a concave or a convex fashion. When this cupping or curling occurs, the affected coated abrasive disc does not lay flat against the back-up pad or support pad. This essentially renders the coated abrasive disc inoperable.